Use Lualatex (#159)

* Use LuaLaTeX instead of pdfTeX

Will allow to use Open Type Fonts in the future

Other less important reasons:
    - Uniform encoding in UTF-8
    - We could maybe use some luaTeX only packages
    - It's more modern, should be better

* Use Tex Gyre Heros as helvetica clone

As it does not seem available by default on the CI machine and might
have some licensing issues.

book/sourdough.sty

@@ -1,16 +1,15 @@
 \ProvidesPackage{sourdough}
-\usepackage[utf8]{inputenc}
 \usepackage{blindtext}
 \usepackage{graphicx}
 \usepackage{booktabs}
 \usepackage{longtable}
 \usepackage{tocbasic}
-\usepackage[T1]{fontenc}
 \usepackage{chemformula}
 \usepackage{chemfig}
 \usepackage{booktabs}
 \usepackage{makecell}
 \usepackage{siunitx}
+\usepackage{fontspec}
 
 \renewcommand\theadfont{\bfseries}
 
@@ -25,8 +24,7 @@
 
 % Fonts for accessibility
 \ifdefined\isaccessible
-    \usepackage{helvet}
-    \renewcommand{\familydefault}{\sfdefault}
+    \setmainfont{TeX Gyre Heros} % Helvetica clone
 \fi
 
 % Kerning in footnotes